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Re: [TowerTalk] Coax as powerline - NOT

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Coax as powerline - NOT
From: "Jim Brown" <>
Date: Wed, 16 Apr 2008 08:14:18 -0700
List-post: <">>
On Tue, 15 Apr 2008 16:53:22 -0700, Jim Lux wrote:

>I don't think you need a neutral (ground*ed* conductor).. two hots and 
>ground will do. (three phase motors have three hots and a ground, for 

That's right, and I noted that in an earlier post -- a 240V circuit in the 
US does NOT require a neutral. But that's not the application -- the 
original poster was talking about 120V, which in North America is phase, 
neutral, and equipment ground. And if the application IS two hots and a 
ground, that's still three wires. 

>And then, is a grounding conductor needed even for an isolated circuit? 
>  There's all sorts of weird rules for isolated circuits, and in this 
>case, the green wire ground is created brand new at the utilization 
>equipment, so the safety issue is addressed (i.e. the case of the 
>equipment is at "bare feet on floor" potential).

I don't pretend to know every exception in the code. BUT -- pro audio and 
video systems make extensive use of what NEC calls isolated ground wiring. 
For a 120V circuit, each outlet REQUIRES a phase (hot), a neutral, and a 
green wire that is not raceway (conduit). AND there must be an additional 
ground conductor that grounds any outlet boxes and raceway. If all raceway 
is conductive, that raceway can serve as its own ground conductor, but any 
lengths of non-conductive raceway (PVC, etc.) must have that fourth 
conductor to bond between the non-conductive segments. 

Building codes (NEC, City of LA, Chicago, etc.) ALL require an equipment 
ground that is sufficiently robust to blow the breaker, AND THAT FOLLOWS 
THE SAME ROUTE as the phase conductor. If the phase conductors are in 
raceway, they must generally be in the same raceway. EARTH has NOTHING to 
do with blowing that breaker. Continuity of the equipment ground back to 
the panel is what blows the breaker, as well as minimizing the inductance 
of the fault current. 

There IS an exception within NEC that permits so-called "balanced power" 
for 120V wiring. It places VERY stringent limitations on such a system and 
how it can be installed, including a requirement that each and every 
outlet have a GFCI. 

It's also important to point out major regional differences in practice, 
all of which are covered by NEC. In many large cities (NY, Chicago, 
others) all electrical wiring is required to be in steel conduit, or use 
an armored cable like BX. In other parts of the country, most homes are 
wired with plastic-jacketed triplets of hot, neutral, and ground, with no 
conduit or other form of protection. Steel conduit has the major benefit 
of shielding wiring from magnetic fields and RFI. 

Several years ago, I wrote a tutorial that addresses Power and Grounding 
for pro audio and video systems. Nearly all of the concepts are 
regulations are directly applicable to home systems and ham stations.

I regularly teach this material in three hours classes at industry trade 
shows. The next one on my schedule is at Infocomm (Lost Wages, June 20). 


Jim Brown K9YC


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